1. Technical Field
The present invention relates generally to the imaging of objects in highly scattering turbid media and, more particularly, to a novel optical backscattering tomographic technique using optical radiation in visible, near infrared (NIR) spectral region for imaging objects in highly scattering turbid media.
2. Description of Related Art
There are many situations in which the detection of objects in a highly scattering turbid medium using backscattered light is highly desirable. For example, backscattered light may be utilized to detect a tumor embedded within tissue, such as breast tissue. Another example is using a laser source and detector located in an aircraft or a satellite to monitor the earth's atmospheric structure, such as cloud distribution, and land and water terrain. This method may also be used to detect hidden objects in a foggy or smoky environment. Various types of microscopes use backscattered light to display the surface image of a medium with high resolution. A confocal arrangement can extend the image to less than 200 .mu.m below the surface.
The conventional Optical Coherent Tomography (OCT) technique, which uses backscattered light, can only image the internal structure of an eye and tissue down to about 600 .mu.m below the skin surface. No clear image of the medium structure in a deeper depth, however, can be formed using the direct backscattered light signals. This is due to multiple light scattering within a medium, which contributes to noise, loss of coherence, and reduces the intensity of light directly backscattered from the hidden object.
Presently, diffusion optical tomography is a widely utilized optical image reconstruction tomographic technique. Examples of references which disclose this technique include: U.S. Pat. No. 5,813,988 to Alfano et al. entitled "Time-Resolved Diffusion Tomographic Imaging In Highly Scattering Turbid Media," which issued Sep. 29, 1998; W. Cai et al., "Time-Resolved Optical Diffusion Tomographic Image Reconstruction In Highly Scattering Turbid Media," Proc. Natl. Acad. Sci. USA, Vol. 93 13561-64 (1996); Arridge, "The Forward and Inverse Problems in Time Resolved Infra-red Imaging," Medical Optical Tomography: Functional Imaging and Monitoring SPIE Institutes, Vol. IS11, G. Muller ed., 31-64 (1993); and Singer et al., "Image Reconstruction of Interior of Bodies That Diffuse Radiation," Science, 248: 990-3 (1993), all of which are incorporated herein by reference.
The conventional diffusion optical tomography method has several disadvantages. For example, the diffusion method only uses diffusive photons which have suffered many scattering events. Therefore, the signals received by detectors are less sensitive to changes in the structure of the turbid medium, which makes it difficult to obtain high-resolution image reconstruction. Furthermore, the diffusion method requires that the source and detector be far enough apart such that diffusion is valid (e.g., larger than 5 l.sub.t where It is the transport mean free path). This leads to non-portable, costly equipment (in contrast to the backscattering arrangement where the sources and the detectors can be set near each other). Indeed, in many important applications it is virtually impossible to arrange the source and the detectors separately. Another disadvantage to this approach is that it requires the simultaneous imaging of a large volume of the medium, which, in many cases, is the entire volume of the turbid medium being tested. When solving the inverse problem, however, due to practical limitations in computation time, the number of voxels (a voxel is a division of the medium) can not be too large since the computation time is proportional to N.sup.2.5-3, where N is the number of voxels. In addition, imaging a large volume leads to a large volume of each voxel and low resolution. Consequently, the resolution obtained by using the conventional diffusion tomography method is on the order of a few centimeters.
The theoretical basis for diffusion tomography is the "diffusion approximation" to the more accurate Boltzmann photon transport equation. The above-mentioned disadvantages associated with diffusion tomography originate from failure of the "diffusion approximation" to describe the early-time migration of photons, which is when the photon distribution is highly anisotropic. Correspondingly, diffusion tomography can not be utilized in a backscattering arrangement, where sources and detectors are arranged near each other and, hence, early-time photon migration plays an important role.